This invention relates to improved water insoluble alkali metal carboxylate salts of starch-acrylonitrile graft copolymers which are produced by saponifying starch acrylonitrile graft copolymers with a base in an aqueous alcoholic medium. The invention represents an improvement over my previous patent U.S. Pat. No. 3,661,815.
Graft copolymers of starch-polyacrylonitrile per se are known as well as are methods for their preparation.
Thus, it is known that acrylonitrile can be grafted on starch using ceric salts as catalysts to form starch-acrylonitrile graft copolymers. See, for example, U.S. Pat. No. 2,922,768. Such graft copolymers can also be prepared by the reaction of acrylonitrile with pre-irradiated starch which is prepared by irradiation of starch with gamma rays or an electron beam. See Reyes, Clark, Comas, Russell, and Rist, Nuclear Applications 6, 509-517 (1969). In such graft copolymers the starch serves as a backbone or building block on which the acrylonitrile is grafted and therefore the starch need be present in only very small proportions with respect to the polyacrylonitrile moiety.
After the starch polyacrylonitrile graft copolymer is produced to make it valuable as a water insoluble material having the ability to absorb large amounts of water, it is saponified. For example, U.S. Pat. No. 3,425,971 is directed to saponification of a graft copolymer in an aqueous potassium hydroxide solution.
Compositions which can readily absorb large amounts of water are valuable for many uses. For example, such substances can be used in manufacturing products such as disposable diapers, tampons, sanitary napkins, paper towels and in numerous hospital or nursing home applications such as bed sheets, application to gauzes or other wound dressing materials, or the like. In addition, such substances can be used in other areas of industry where water absorbing and retention is important. For example, it can be mixed with wood pulp or the like, or added to a soil in order to increase the water retention capability of the soil.
Ideally, water absorbing compositions should not only absorb as much water as possible and therefore be highly efficient in this respect, it must also not have the property of "thinning" after exposure to solutions that have a high salt and enzyme content. This is a very important consideration in that many of the body fluids which such a composition will come in contact with when used in disposable diapers, or sanitary napkins, have both a high salt content and a high enzyme content. Appreciable contact with both salts and enzymes will allow many water absorbing copolymers to become less viscous and even run with the water retention ability reduced after a short period of time. Thus, it is very important that not only the composition have high water absorbence capability, but it must also not be adversely affected so that it will thin upon contact with salt and enzyme containing liquids.
In addition, an ideal composition must be one which will have the ability to adhere to fibers of, for example, paper towels or other material upon which it is coated. Most graft copolymers, especially of starch polyacrylonitrile do not have a property of good adherence to a substrate material.
In addition, primarily for aesthetic purposes so that they will not interfere with the looks and quality of a product in which such water absorbing compositions are applied, a good polymer should provide a clear product when in dispersion form.
The object of this invention is to provide a starch polyacrylonitrile graft copolymer which has an unusually high water absorbence capacity, i.e., capable of absorbing from about 800 to as much as 1000 times its weight of water.
Another object of this invention is to provide a starch polyacrylonitrile graft copolymer which is capable of absorbing up to as much as 100 times or greater, of its weight of body fluids.
Still another object of this invention is to provide a starch polyacrylonitrile graft copolymer which has significantly improved characteristics in terms of improved performance in the presence of ionic and enzyme containing fluids in that the polymer paste does not thin upon standing.
A still further object of this invention is to provide a graft copolymer which, when in wet form, will form a paste like material which has good adhesion properties to a substrate material.
Yet another object of this invention is to provide a graft copolymer which has a high acrylonitrile content and yet which in a wet form dispersion is a clear product which will not interfere with the aesthetic characteristics of a substrate material to which it is applied.
The method and means of accomplishing at least all of the above stated objects will become apparent from the detailed description of the invention which follows.